Growing official concern about pollution and air quality, especially in urban areas, has led to the adoption of emission standards and rules in many jurisdictions.
Such emission standards often set requirements which define acceptable limits for exhaust discharges from vehicles equipped with combustion engines. These standards often regulate, for example, levels of discharge of nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) and particles from most types of vehicles.
The endeavour to meet such emission standards has led to ongoing research with a view to reducing emissions by means of post-treatment (cleaning) of the exhaust gases which arise from combustion in a combustion engine.
One way to post-treat exhaust gases from a combustion engine is a so-called catalytic cleaning process, so vehicles, including large means of transport powered by combustion engines, are usually also provided with at least one catalyst.
Post-treatment systems may also, either alternatively or in combination with one or more catalysts, comprise other components, e.g. particle filters. There are also cases where particle filters and catalysts are integrated with one another.
Combustion of fuel in the cylinders of a combustion engine results in the formation of soot particles. Particle filters are used to capture these soot particles, and work in such a way that the exhaust flow is led through a filter structure whereby soot particles are captured from the passing exhaust flow and are stored in the particle filter.
The particle filter fills with soot progressively during vehicle operation, and has to be emptied of it sooner or later, which is usually achieved by so-called regeneration.
Regeneration involves the soot particles, which mainly consist of carbon particles, being converted to carbon dioxide and/or carbon monoxide in one or more chemical processes, and may in principle be effected in two different ways. One way is regeneration by so-called oxygen (O2) based regeneration, also called active regeneration. In active regeneration, carbon is converted by oxygen to carbon dioxide and water.
This chemical reaction requires relatively high particle filter temperatures for desired reaction rates (filter emptying rates) to be achieved at all.
Instead of active regeneration, it is possible to apply NO2 based regeneration, also called passive regeneration. In passive regeneration, nitrogen oxide and carbon monoxide are formed by a reaction between carbon and nitrogen dioxide. The advantage of passive regeneration is that desired reaction rates, and hence the rate at which the filter is emptied, can be achieved at significantly lower temperatures.
Irrespective of whether active or passive regeneration is applied, it is nevertheless important that it be conducted in an effective way so that regeneration of a particle filter can be done within a reasonable time.